This invention relates to a DC Superconducting Quantum Interference Device (hereinafter referred to as "DC-SQUID").
FIG. 6 is a plan view of a DC-SQUID known in the prior art. The DC SQUID comprises an input coil 1 for transmitting an external magnetic field signal as a signal current from a detection coil, a washer coil 2 to which the signal current is input as a signal magnetic flux, two Josephson junction devices 3 for converting the signal magnetic flux transmitted to the washer coil 2 to a signal voltage, shunt resistors 4 which are connected to the Josephson junction devices in parallel respectively to extinguish the hysteresis of a current voltage characteristic, a dampening resistor 5 which is connected to the Josephson junction devices in series, and a feedback modulation coil 6 for transmitting a feedback and modulation current from an external control circuit. The feedback modulation coil 6 is arranged near one turn at the upper portion or lower portion of the washer coil 2.
In order to obtain a high coupling coefficient k(iw) and a small parasitic capacitance, the inner diameter of the washer coil 2 is set to be large to make the self inductance Lw of the washer coil 2 large.
In general, the self inductance Lm of the feedback modulation coil 6 satisfies the following equation: EQU Lm=2n.times.Lw+Ls (1)
where Ls is the strip line inductance of the feedback modulation coil 6 n is the turn number of the feedback coil, and Lw is the self inductance of the washer coil 2.
Further, the mutual inductance Mms between the washer coil 2 and the feedback modulation coil 6 satisfies the following equation: EQU Mms=k(ms).times..sqroot.(Lw.times.Lm) (2)
where k(ms) is the coupling coefficient between the washer coil 2 and the feedback modulation coil 6.
When the feedback modulation coil 6 is designed to have the one-turn arrangement at the upper portion or lower portion of the washer coil 2 like the prior art, n=1 and k(ms)=1. Therefore, the following equation is approximately satisfied from the equations (1) and (2): EQU Mms=Lw (3)
In general, the magnetic-field sensitivity .alpha. of the SQUID and Mms are inversely proportional to each other. Therefore, since Mms is larger as the self inductance is larger, the magnetic-field sensitivity .alpha. is reduced and the SN ratio is deteriorated. Further, magnetic flux noise .PHI.n which intersect to the washer coil 2 through the feedback modulation coil 6, such as an environmental noise and an RF noise from the external control circuit are intensified, and thus there occurs a problem that a magnetic flux-voltage conversion efficiency of the SQUID is reduced.